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Abstract

We investigate the use of a superconducting nano-detector as a novel near-field probe. In contrast to conventional scanning near-field optical microscopes, the nano-detector absorbs and detects photons in the near-field. We show that this absorption-based probe has a higher collection efficiency and investigate the details of the interaction between the nano detector and the dipole emitter. To this end, we introduce a multipole model to describe the interaction. Calculations of the local density of states show that the nano-detector does not strongly modify the emission rate of a dipole, especially when compared to traditional metal probes.

D. W. Lynch and W. R. Hunter, “Comments on the optical constants of metals and an introduction to the data for several metals,” in Handbook of Optical Constants of Solids I, E. D. Palik, ed. (Academic, 1998).

C. F. Bohren and D. R. Huffman, “Particles small compared with the wavelength,” in Absorption and scattering of light by small particles. (Wiley & Sons, 1983).

D. W. Lynch and W. R. Hunter, “Comments on the optical constants of metals and an introduction to the data for several metals,” in Handbook of Optical Constants of Solids I, E. D. Palik, ed. (Academic, 1998).

Figures (6)

Absorption cross section for a NbN sphere close to a GaAs substrate, as compared to a Ag sphere. Calculations are done at λ = 1.0 μm, as a function of sphere radius. The inset shows a sphere with a radius of a at distance z = 10 nm from the semi-infinite substrate. The particle is excited by an external electric field E parallel to the interface. In the Rayleigh limit (a/λ << 1), the absorption cross section is much larger than the scattering cross section. Because of the larger imaginary part of the dielectric constant of NbN, the absorption cross section of a NbN sphere is 10 times larger than that of a Ag sphere.

Comparison of the absorption cross section for a square and spherical NbN nano-detector as a function of size a at λ = 1.0 μm. Simulations are used to calculate the absorption of a square detector, both at λ = 1.0 μm. In the optical near-field (a/λ <<1), the absorption of a NbN square is higher than that of a sphere. The inset shows a schematic picture of a realistic NbN nano-detector: a 4 nm thick NbN film is grown on a GaAs substrate. A bowtie shape with a nanoscale square constriction serving as the detector active area is patterned in the film.

Physical model used in FDTD simulations of detector sensitivity and resolution. The NbN nano-detector is located above a dipole emitter (point source) with a dipole moment along X-direction. The emitter (λ = 1.0 μm) is placed on the semi-infinite GaAs substrate. Charge areas are induced inside the nano-detector and in the substrate underneath the emitter.

Calculated absorption of a point dipole as a function of distance between the nano-detector and the substrate. Calculations are done for a wavelength λ = 1.0 μm with the radiation power of the dipole source fixed at 1 W. The FDTD results are compared with two models: a dipole (a) and mutipole (b) model.

Normalized life time of an emitter close to a 4 nm thick metal film as a function of distance from the substrate. Calculation are shown for a NbN, Au, Ag and Al film at λ = 1.0 μm. In the near field (distance < 50 nm), the emitter close to NbN behaves differently from those close to the other three metal films. The inset shows the variations in emitter lifetime for larger distances.

Local density of states (LDOS) at the position of the emitter as a function of frequency for different metal substrates. Results are shown for a 4 nm thick NbN film and a 4 nm thick Ag film on a semi-infinite GaAs substrate as shown in the inset. The distance between the emitter and the film is set to 5 nm and 10 nm which are in the near-field of the emitter leading to a strong coupling to surface plasmon modes of the Ag film.